By Nancy Gibbs and Michael Duffy
February 12, 2015

Here is a Classic Conundrum of cause and effect: the men who survive the crushing pace (not to mention lethal diet) of multiple U.S. presidential campaigns and go on to hold one of the most stressful jobs in the world also have a habit of outliving the rest of us.

In the fall of 2012, Jimmy Carter, now 90, took his place in history as the President who had lived the longest after leaving the White House–31 years and 231 days out of office, breaking the record of Herbert Hoover, who died in 1964. Carter left the White House in January 1981, went back to Georgia and proceeded to teach, improve his Spanish, paint, write poetry, win the Nobel Peace Prize and write 21 books about, among other things, how to find a second career. He is rather typical. Ronald Reagan lived until 93. So did Gerald Ford. George Bush the elder, like Carter, is 90. Even in the 19th century, when the average man died at age 47, U.S. Presidents lived an average of 69 years, and John Adams made it to 90. Granted, the presidential demographic typically enjoys access to better nutrition, health care and living conditions. Yet these men also knew pressure that few of us can imagine, and stress is a proven toxin.

So does the presidency endow people with some special life force, or do they share some quality that helps get them to the White House in the first place? Is there something about holding the office that forces men–and presumably one day women–to live a healthy lifestyle rather than just aspire to it?

For starters, there is constant vigilance. Ignoring troubling symptoms is not an option for someone who has a doctor following him virtually everywhere he goes; medical teams are steps away at all times. Even when Presidents return to private life, they are shadowed by Secret Service details, albeit smaller ones. Among those agents, an EMT is always on duty. Think of it as a retirement benefit.

At least since the mid-1970s, nearly every President has been devoted to some kind of regular exercise. Ford swam and skied. Carter jogged almost daily. Reagan chopped brush and lifted weights. Bush the younger took to biking when his knees put a halt to running. If some of that recreation was done for public relations purposes, most Presidents have come to rely on it for private sustenance. (Not everyone got the memo: as President, Bill Clinton may have been conspicuously photographed in his jogging shorts, but he typically relaxed in the office by reading, doing crosswords and chewing on cigars, a pattern that probably helped land him in New York–Presbyterian Hospital for bypass surgery in 2004 and another surgery six years later. He is now a part-time vegan.)

The elder Bush, who as President was known to try three or four different sports in a single day, still takes exercise to extremes and jumped out of airplanes with Army skydivers at ages 80, 85 and 90. “I want people at my age to know they don’t have to slow down,” he once told us. Last summer the elder Bush had young aides haul his wheelchair out on the dock of his Maine seaside compound so he could bomb around the North Atlantic on his speedboat.

There’s body, and then there’s mind. We all may know we need to manage our stress, but for a sitting President this is imperative, a consistent part of the advice they give one another. Be sure to rest. Take your vacations. Use Camp David. After the hard-fought 1960 election, Richard Nixon and John F. Kennedy met in Key Biscayne, Fla., where Nixon made an unsolicited promise. I may criticize your policies, he told Kennedy, but “of one thing I can assure you: I shall never join in any criticism of you, expressed or implied, for taking time off for relaxation. There is nothing more important than that a President be physically, mentally and emotionally in the best possible shape to confront the immensely difficult decisions he has to make.”

For many Presidents, stress acts as a force multiplier. The toll stress takes, research has shown, depends on how it is viewed. What is normally harmful becomes helpful when it is treated as a fact of life or a chance to learn. The more powerful a person is, the more in control, the better the odds he has learned to use stress to his advantage. Clinton aides flaunted his mantra like a bumper sticker: “That which doesn’t kill him only makes him stronger.” For people with that kind of resilience–sometimes called adaptive competence–stress can correlate with a longer life.

Out of office, the challenge changes. Presidents tend to be not just type-A but triple-A personalities, guys who don’t spend a lot of time lounging around checking their Facebook feeds (although Clinton and Bush the elder both tweet). Slowing down isn’t something they really want to do. “When I got out of the White House,” Carter recalled, “I had a life expectancy of 25 [more] years, and so I needed to figure out how to use it.”

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Former Presidents are particularly well positioned to do good: to engineer an immense humanitarian rescue effort, as Hoover did in the years after World War II, or to promote reform and democracy, as Ford and Carter did as unlikely partners. Clinton launched his Global Initiative, while George W. Bush has focused on veterans. Engaging in meaningful work also correlates closely with longevity–and modern Presidents have typically made it their mission to leverage their fame for a cause they believe in. So, in psychological terms, they settle in for the long haul.

Of course, ex-Presidents have something else to keep them going: a need to burnish their reputation for history, particularly if their time in office didn’t go exactly as they had planned. Most of us are not quite as likely to have accumulated as many regrets and scars, nor are we in as strong a position to do something about them. Correcting–or whitewashing–the record on a global scale probably helps keep the former Presidents alive a little longer, if only because there is often so much work to be done. That can be a campaign that never ends, a second life’s work.

It may even be that unloved Presidents have an edge in this area. Few wept when Harry Truman left Washington in 1953, ceding the White House to the wildly popular Dwight Eisenhower. But Truman lived another 19 years, and his reputation improved annually. Even Nixon, who resigned in 1974, lived two more decades, writing books, opening a think tank and driving his successors more or less crazy. He was determined to have the last word, which might be a useful longevity strategy. Asked once how he had survived all the criticism aimed at him during the Depression, Hoover said simply, “I outlived the bastards.”

Finally, there’s the kind of legacy you can read about in books, and then there’s the kind any fool can see. And so as 2015 unfolds, it’s important to note that three of the four current ex-Presidents may have something else to live for now. Clinton’s wife Hillary is trying to become the 45th President, and so is Bush son and brother Jeb. Aides to the elder Bush privately admit that the prospect of seeing a second son run for the White House helps keep him going.

And nothing gets the blood pumping like a little competition among friends.

This appears in the February 23, 2015 issue of TIME.

Contact us at editors@time.com.

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